1. Field of Art
The disclosure generally relates to the field of electronic paper displays. More particularly, the invention relates to systems and methods for displaying a page transition on electronic paper displays.
2. Description of the Related Art
Several technologies have been introduced recently that provide some of the properties of paper in a display that can be updated electronically. Some of the desirable properties of paper that this type of display tries to achieve include: low power consumption, flexibility, wide viewing angle, low cost, light weight, high resolution, high contrast and readability indoors and outdoors. Because these types of displays attempt to mimic the characteristics of paper, they are referred to as electronic paper displays (EPDs) in this application. Other names for this type of display include: paper-like displays, zero power displays, e-paper and bi-stable displays.
A comparison of EPDs to Cathode Ray Tube (CRT) displays or Liquid Crystal Displays (LCDs) reveal that in general, EPDs require less power and have higher spatial resolution; but have the disadvantages of slower update rates, less accurate color control, and lower color resolution. Many electronic paper displays were previously only grayscale devices. Color EPDs are becoming available although often through the addition of a color filter, which tends to reduce the spatial resolution and the contrast.
The key feature that distinguishes EPDs from LCDs or CRTs is that EPDs can maintain an image without using power. They are sometimes referred to as “bi-stable” because black or white pixels can be displayed continuously and power is only needed to change from one state to another. However, some devices are stable at multiple states and thus support multiple colors without power consumption. EPDs are also typically reflective rather than transmissive. Thus they are able to use ambient light rather than requiring a lighting source in the device. Various technologies have been developed to produce EPDs. Depending on the technology used, such displays are sometimes called electrophoretic displays, electro-wetting displays, cholesteric LCD (Ch-LC). Techniques have also been developed to produce EPDs by embedding organic transistors into flexible substrates.
The luminance or color of a pixel in a traditional LCD display depends on the voltage currently being applied at the given point, with a given voltage reliably corresponding to a specific luminance. The luminance or color of a pixel in a bistable display, on the other hand, typically changes as voltage is applied. For example, in some bistable displays applying a negative voltage to a pixel makes it lighter (higher luminance) and a positive voltage makes it darker. The higher the voltage and the longer or more times that voltage is applied, the larger the change in luminance. This has two implications for driving such displays. First, electronic paper displays are typically controlled by applying a sequence of voltages to a pixel instead of just a single value like a typical LCD. These sequences of voltages are sometimes called waveforms. The second implication is that the control signals used to drive a pixel depend not only on the optical state the pixel is being driven to, but also on the optical state it is being driven from. Depending on the display technology, other factors may also need to be taken into consideration when choosing the waveform to drive a pixel to a desired color. Such factors can include the temperature of the display, optical state of the pixel prior to the current optical state, and dwell time (i.e. the time since the pixel was last driven). Failure to take these factors into account can lead to faint remnants of images that have supposedly been erased still being visible, a visual artifact known as ghosting. Some displays also have additional requirements that must be met to avoid damaging the display, such as the requirement that waveforms be DC balanced.
To handle these issues, some controllers for driving the displays are configured like an indexed color-mapped display. The framebuffer of these electronic paper displays includes an index to the waveform used to update that pixel instead of the waveform itself. Whenever the optical state of a pixel is to be changed, the index of the appropriate waveform is chosen based on at least some of the factors listed above, and the pixel's location in the frame buffer is set to that index. Some displays will encode some factors (such as a pixel's current and desired optical state) in the waveform index and then choose which waveform table to use when updating a set of pixels based on other factors (such as temperature).
One problem with the above technique is that it typically takes longer to compute which waveform to apply to a pixel than it does to perform the corresponding operation on a conventional CRT or LCD display. This can lead to a considerable latency between when an application requests a new image be displayed and when the image actually appears. For example, an EPD using a prior art controller can take on the order of half a second to calculate new pixel values for a 1200×825 display. The latency can be improved with faster or additional hardware, but only with increased cost and power consumption. To some extent the latency can also be reduced by simplifying the calculation, for example by ignoring secondary factors such as dwell time and pixel history (prior displayed colors for the pixel) prior to the current optical state, but this can result in increased ghosting.
While current update times are generally sufficient for the page turning needed by electronic books, they are problematic for interactive applications that emulate page transitions or page flipping at higher speeds. A user may tolerate waiting for a second or two for transitioning between two pages when the user spends a few minutes reading each page. However, when the user wants to flip through numerous pages successively without spending more than a few seconds on each page such as to find a section, illustration or particular part of a larger document, the transition time of half a second between pages becomes unacceptable.
There have been attempts in the prior art to solve page flipping problem described above in the previous paragraph. While those attempts approximate something like page transitions, they have certain shortcomings. One particular problem is that precise timing is required for the block copying of data such that the copying is ahead of the LCD controller. Managing this timing can be problematic if the processor is under heavy load for other activities. Another problem is that the needed data files for fast page flipping can be very large; for example, just under 1 MB per page for a display that is 1200×827 pixels. Finally, the prior art approaches required a block copy for each page transition. This means that writing about a megabyte to the frame buffer for each page transition, which translates to one megabyte every 80 ms for 12.5 pages per second transition rate. Thus there is a significant burden on computing resources that can be used for other applications.